Alzheimer’s patients characteristically get testy as their memory fails and they are more and more at a loss to communicate. Their personalities change, sometimes a lot.
Biden’s seems to have not changed. Just that whatever restraint he formerly had, which wasn’t much, has slipped away.
Their brains resort back to searching long term memory. Old emotions come forward. Anger is common. So is thinking the past is now.
Biden breaks down on the drugs they pump him with by doing things he did constantly for decades campaigning, giving speeches, etc. Thus he turns right after a speech to shake hands.